The present invention relates to a device and method that utilize segmental bioimpedance for monitoring and controlling physiologic parameters of a dialysis patient.
Accurate assessment of a dialysis patient""s hydration status and prediction of dry body weight (DW or dry weight) is a major problem in the clinical management of the dialysis patient. In both hemodialysis and peritoneal dialysis patients, dry weight is the target weight at the end of dialysis treatment which best reflects removal of excess water from the body. In clinical practice, estimation of DW is an imprecise undertaking, and depends to a large extent on the treating physician""s interpretation, based on his or her medical experience and familiarity with the particular patient""s condition, of clinical symptoms and signs such as changes in the blood pressure, pulse, and weight of the patient. The correct interpretation of such signs and symptoms is complicated by the fact that the pre-treatment body weight varies for each treatment, the amount of excess fluid is not constant and the amount of fluid that can or should be removed from any particular patient during any particular dialysis treatment may be limited by an individual""s cardiovascular tolerance, often manifested by clinical signs and symptoms, such as pretibial edema, dyspnea, cramps and/or a decline in blood pressure. Alternatively, an overestimation of the amount of fluid to be removed may result in potentially avoidable symptoms, unnecessarily lengthy dialysis treatments and often prolonged stays at the dialysis facility. Therefore, over- or underestimation of DW will significantly affect both the efficiency of dialysis treatment and patients"" quality of life.
Bioelectrical impedance analysis (BIA) has been recognized as a noninvasive and simple technique to measure body hydration and hydration status (i.e. over-, under- or normal hydration) of subjects for more than twenty years. There is substantial literature on using BIA for the study of dry weight. Kouw et al proposed a method to measure changes in regional conductivity, and then to measure regional extracellular volume (ECV) and intracellular volume (ICV) by BIA. See, P. M. Kouw, et al, Assessment of post-dialysis dry weight: an application of the conductivity measurement method. Kidney Int. 41:440-444,1992. However, Kouw""s method cannot be used to measure interstitial fluid alone as it does not distinguish between interstitial fluid and plasma, both of which make up the ECV compartment. Piccoli published a method of BIA vector analysis which uses the ratio of resistance to reactance to identify dry weight. While this technique could be used to compare the subjects"" body hydration, it is unable to predict individual patient""s dry weight because of the significant variation in measured values. See, Piccoli A: Identification of operational clues to dry weight prescription in hemodialysis using bioimpedance vector analysis. Kidney Int. 5 3:1036-1043,1998.
Recently, there have been increased numbers of dry weight studies using blood volume (BV) measurements. See, for example, J. P. de Vries et al, Non-invasive monitoring of blood volume during hemodialysis: Its relation with post-dialytic dry weight. Kidney Int 44:851-854,1993, and J. K. Leypold, et al, Determination of circulating blood volume by continuously monitoring hematocrit during hemodialysis. J. Am. Soc. Nephrol. 6:214-219,1995. Blood volume measurement is a noninvasive technique that can be used to indicate water concentration in blood, i.e. hematocrit, during hemodialysis, but it cannot be used to directly determine dry weight because changes in blood volume are mainly dependent on the rate of vascular refilling which, in part, is independent of body hydration. See, e.g., J. K. Leypoldt, et al, Evaluating volume status in hemodialysis patients. Adv. Ren. Replace. Ther. 5:64-74,1998. On the other hand, since a change in the hematocrit level may alter conductivity in the blood during dialysis, it is difficult to obtain information about tissue hydration by either traditional bioelectrical impedance analysis or blood volume analysis. To date, a major problem has been how to measure resistivity of blood and tissue separately, in order to estimate the fluid volume in the intravascular compartment and the interstitial compartment, respectively.
Thus, there is a need for a precise, easily used and operator independent method for determining the hydration status of a dialysis patient, identifying or predicting the dry weight of such a patient and calculating the amount of fluid that should be removed during a dialysis session. In addition, there is a need for a method of controlling dialysis in response to a patient""s hydration status.
The present invention includes a method for determining the hydration status of a dialysis patient comprising the steps of measuring the resistivity of a body segment of the patient, correlating the measured resistivity with predetermined normal dry weight values, and deriving the patient""s hydration status. Optionally the resistivity of the interstitial fluid in the body segment is measured to derive the patient""s hydration status. In one embodiment, the resistivity of the body segment is determined while applying a pressure of at least about systolic blood pressure, optionally from about 120 mmHg to about 240 mmHg. The body segment can be a limb segment, preferably a thigh segment.
Included, is a method for determining a hemodialysis patient""s dry weight comprising the steps of periodically measuring the resistivity of a body segment during hemodialysis; comparing successive resistivity measurements; and identifying the patient""s dry weight when a substantially constant resistivity is reached. Optionally, resistivity is measured from about every 5 minutes to about every 20 minutes during hemodialysis, preferably about every 10 minutes during hemodialysis. In one embodiment the resistivity of the body segment is measured at a pressure of at least about systolic blood pressure, optionally from about 120 mmHg to about 240 mmHg.
The present invention includes a method for dialysing a patient to the patient""s dry weight that comprises measuring the resistivity of a body segment of the patient, correlating the measured resistivity with predetermined normal dry weight values, deriving the patient""s hydration, and continuing hemodialysis until the resistivity of the body segment correlates with the predetermined normal dry weight values, preferably measuring the resistivity of the body segment at a pressure of at least about systolic blood pressure.
Also provided is a method for hemodialysing a patient to the patient""s dry weight comprising the steps of periodically measuring the resistivity of a body segment during hemodialysis, comparing successive resistivity measurements, and discontinuing hemodialysis when a substantially constant resistivity is reflected. Preferably, the resistivity of the body segment is measured at a pressure of at least about systolic blood pressure. In this embodiment, the resistivity of the body segment is measured from about every 5 minutes to about every 20 minutes during hemodialysis.
The present invention also provides a method of monitoring the heart rate of a hemodialysis patient comprising the steps of determining a time interval between two successive bioimpedance wave peaks and multiplying the reciprocal of the time interval by 60 to obtain the heart rate, and a method of calculating the cardiac output of a patient in need thereof comprising the steps of measuring the stroke volume in an arm segment by bioimpedance analysis, substantially simultaneously measuring the stroke volume in an ipsalateral leg segment by bioimpedance analysis, summing the stroke volume in the arm segment and the stroke volume in the leg segment, and multiplying the sum by twice the heart rate to obtain the cardiac output. Preferably, the stroke volume of the arm segment is calculated by applying an external maximum pressure to the arm segment and determining the change in blood volume in the arm segment between the point of maximum pressure and the point at which no external pressure is applied divided by the number of heart beats between the two points in time, and the stroke volume of the leg is calculated by applying an external maximum pressure to the leg segment and determining the change in blood volume in the leg segment between the point of maximum pressure and the point at which no external pressure is applied divided by the number of heart beats between two points in time.
Included is a device for controlling a hemodialysis machine comprising a bioimpedance analysis measurement unit in electrical communication with a hemodialysis machine, an electrical output means that is in electrical communication with the bioimpedance analysis measurement unit and that is attachable to a body segment, the electrical output means is adapted to apply electrical current to the body segment, an electrical input means that is in electrical communication with the bioimpedance analysis measurement unit and is attachable to a body segment, the electrical input means being adapted to receive the current transmitted through the body segment and transmit the same to the bioimpedance analysis measurement unit. The bioimpedance analysis measurement unit is adapted to determine body segment resistivity based on the current transmitted through the body segment and the bioimpedance analysis measurement unit provides feedback to the hemodialysis machine in response to the body segment resistivity. In one preferred embodiment, the device includes means for applying pressure to the body segment, the pressure applying means is in electrical communication with the bioimpedance analysis measurement unit. Optionally, the pressure applying means includes a pressure cuff that is adapted to encircle the body segment. Preferably, the electrical output means includes at least two injector electrodes, the electrical input means includes at least two measurement electrodes. The injector electrodes and the measurement electrodes are secured to the pressure cuff. Optionally, the pressure cuff includes at least one conductive band with opposing ends and a conductive plate positioned adjacent one of the ends of the conductive band, the conductive band extends substantially the length of the pressure cuff. The conductive plate is arranged to electrically contact the conductive band at a point along the length of the same wherein the distance between the conductive plate and the point of contact of the conductive band is substantially equal to the circumference of the body segment, and wherein the bioimpedance analysis measurement unit is adapted to electrically determine body segment circumference based on the distance between the end of the band adjacent to the plate and the point of contact of the plate along the length of the band.
One embodiment is a device for monitoring hydration status in a hemodialysis patient comprising a bioimpedance analysis measurement unit, an electrical output means, optionally comprising at least two injector electrodes, being in electrical communication with the bioimpedance analysis measurement unit and being attachable to a body segment, the electrical output means being adapted to apply electrical current to the body segment, an electrical input means, optionally comprising at least two measurement electrodes, being in electrical communication with the bioimpedance analysis measurement unit and being attachable to a body segment, the electrical input means being adapted to receive the current transmitted through the body segment and transmit the same to the bioimpedance analysis measurement unit. The bioimpedance analysis measurement unit is adapted to determine body segment resistivity based on the current transmitted through the body segment. Optionally the device includes a hemodialysis machine. Optionally the device includes means for applying pressure to the body segment, optionally a pressure cuff. The pressure applying means being in electrical communication with the bioimpedance analysis measurement unit.
One embodiment of the device includes a pressure cuff with at least one conductive band with opposing ends and a conductive plate positioned adjacent one of the ends of the conductive band. The conductive band extends substantially the length of the pressure cuff and is arranged to electrically contact the conductive band at a point along the length of the same wherein the distance between the conductive plate and the point of contact of the conductive band is substantially equal to the circumference of the body segment, and wherein the bioimpedance measurement unit is adapted to electrically determine body segment circumference based on the distance between the end of the band adjacent to the plate and the point of contact of the plate along the length of the band.
The present invention includes a device for calculating cardiac output through bioimpedance measurements of a patient comprising a bioimpedance measurement unit, a first electrical output means being in electrical communication with the bioimpedance analysis measurement unit and being attachable to an arm segment, the first electrical output means being adapted to apply electrical current to the arm segment, a second electrical output means being in electrical communication with the bioimpedance analysis measurement unit and being attachable to a leg segment, the second electrical output means being adapted to apply electrical current to the leg segment, a first electrical input means being in electrical communication with the bioimpedance analysis measurement unit and being attachable to an arm segment, the electrical input means being adapted to receive the current transmitted through the arm segment and transmit the same to the bioimpedance analysis measurement unit, a second electrical input means being in electrical communication with the bioimpedance analysis measurement unit and being attachable to a leg segment, the electrical input means being adapted to receive the current transmitted through the leg segment and transmit the same to the bioimpedance analysis measurement unit, a first pressure applying means for applying a maximum pressure to the arm segment, the first pressure applying means being in electrical communication with the bioimpedance analysis measurement unit, a second pressure applying means for applying a maximum pressure to the arm segment, the second pressure applying means being in electrical communication with the bioimpedance analysis measurement unit, means for selectively electronically connecting the bioimpedance analysis measurement between the first electrical input and output mans and the second electrical input and output means, and wherein the bioimpedance analysis measurement unit is adapted to selectively measure stroke volume in the arm and leg segments by bioimpedance analysis.
Other objects, features and advantages of the invention will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the drawings.